Make Ice Cream in a Baggie
From Anne Marie Helmenstine, Ph.D.,
Your Guide to Chemistry.
Freezing Point Depression and Colligative Properties
Materials











1/2 cup milk
1/2 cup whipping cream (heavy cream) or (substitute 4 Tbsp. half-n-half)
1/4 cup sugar
1/4 teaspoon vanilla or vanilla flavoring (vanillin)
1/2 to 3/4 cup sodium chloride (NaCl) as table salt or rock salt
2 cups ice
1-quart ZiplocTM bag
1-gallon ZiplocTM bag
themometer
measuring cups and spoons
cups and spoons for eating your treat!
Procedure
1. Add 1/4 cup sugar, 1/2 cup milk, 1/2 cup whipping cream, and 1/4
teaspoon vanilla to the quart ziplocTM bag. Seal the bag securely.
2. Put 2 cups of ice into the gallon ziplocTM bag.
3. Use a thermometer to measure and record the temperature of the ice in
the gallon bag.
4. Add 1/2 to 3/4 cup salt (sodium chloride) to the bag of ice.
5. Place the sealed quart bag inside the gallon bag of ice and salt. Seal the
gallon bag securely.
6. Gently rock the gallon bag from side to side. It's best to hold it by the top
seal or to have gloves or a cloth between the bag and your hands
because the bag will be cold enough to damage your skin.
7. Continue to rock the bag for 10-15 minutes or until the contents of the
quart bag have solidified into ice cream.
8. Open the gallon bag and use the thermometer to measure and record the
temperature of the ice/salt mixture.
9. Remove the quart bag, open it, serve the contents into cups with spoons
and ENJOY!
Explanation
Ice has to absorb energy in order to melt, changing the phase of water from a
solid to a liquid. When you use ice to cool the ingredients for ice cream, the
energy is absorbed from the ingredients and from the outside environment (like
your hands, if you are holding the baggie of ice!). When you add salt to the ice, it
lowers the freezing point of the ice, so even more energy has to be absorbed
from the environment in order for the ice to melt. This makes the ice colder than it
was before, which is how your ice cream freezes. Ideally, you would make your
ice cream using 'ice cream salt', which is just salt sold as large crystals instead of
the small crystals you see in table salt. The larger crystals take more time to
dissolve in the water around the ice, which allows for even cooling of the ice
cream.
You could use other types of salt instead of sodium chloride, but you couldn't
substitute sugar for the salt because (a) sugar doesn't dissolve well in cold water
and (b) sugar doesn't dissolve into multiple particles, like an ionic material such
as salt. Compounds that break into two pieces upon dissolving, like NaCl breaks
into Na+ and Cl-, are better at lowering the freezing point than substances that
don't separate into particles because the added particles disrupt the ability of the
water to form crystalline ice. The more particles there are, the greater the
disruption and the greater the impact on particle-dependent properties (colligative
properties) like freezing point depression, boiling point elevation, and osmotic
pressure. The salt causes the ice to absorb more energy from the environment
(becoming colder), so although it lowers the point at which water will re-freeze
into ice, you can't add salt to very cold ice and expect it to freeze your ice cream
or de-ice a snowy sidewalk (water has to be present!). This is why NaCl isn't
used to de-ice sidewalks in areas that are very cold.
http://chemistry.about.com/cs/howtos/a/aa020404a.htm
Teacher Notes: For ALL Chemistry classes (~250 kids)
4 – 40 lb bags of rock salt
4 @ $5.00
8 – gallons whole milk
8 @ $3.00
6 – half-n-half cream
6 @ $2.50
3 – imitation vanilla flavoring
3 @ $0.99
LOTS of ICE
(Cafeteria)
400 plastic spoons
Portion control cups to eat ice cream
$20.00
$24.00
$15.00
$ 3.00
FREE
Also requires small ziploc bags (one/2 students) and gallon ziplock bags (~20 bags)
and very lage 2.5 gallon bags (~12 bags).
Clean-up requires lots of rinsing of countertops with warm water to remove ALL
salt residue. Leave 15 minutes to clean tables and desk surfaces.
We did lab day before spring break. Fewer students were present.
Countertop Chemistry Experiment 15
Ice Cream
Adding a solute to a solvent lowers the freezing point of that solvent. This change in freezing
point is referred to as a colligative property. In this experiment, you will use the lowered freezing
point of water to chill another mixture (ice cream) to the solid state.
Materials
Subsitutions
1 quart Ziploc™ bag
1 gallon Ziploc™ bag
1/2 cup milk
1/2 cup whipping cream
1/4 cup sugar
1/4 teaspoon vanilla flavoring
sodium chloride
rock salt
ice
thermometer
measuring cups (1, 1/2, and 1/4 cups)
Styrofoam™ cups
plastic spoons
Procedure
1. Into a one-quart Ziploc™ bag, place 1/4 cup sugar, 1/2 cup milk, 1/2 cup
2.
3.
4.
5.
6.
7.
8.
9.
whipping cream, and 1/4 teaspoon vanilla (4-hydroxy-3-methoxybenzaldehyde).
Securely seal the bag and mix well.
Into a one-gallon Ziploc™ bag, place 2 cups of ice.
Using the thermometer, measure and record the temperature of the ice.
Add between 1/2 and 3/4 cups of sodium chloride to the gallon bag.
Place the sealed quart bag into the gallon bag. Close the larger bag securely.
Holding the large bag by the top seal, gently rock the bag from side to side. Do
not hold the bag in your hands—it will be cold enough to cause tissue damage to
your hands.
Continue rocking the bag until the contents of the quart bag have solidified (1015 minutes).
Measure the temperature of the salt/ice mixture in the gallon bag and record the
temperature.
Remove the frozen contents from quart bag into Styrofoam™ cups. Consume the
contents of the cups.
Data and Observations
Initial temperature of ice _____
Final temperature of ice mixture _____
Change in temperature _____
Questions
1. Why is sodium chloride added to the ice?
2. Why are large crystals of sodium chloride used instead of small crystals
3. Why is sodium chloride placed on icy patches on highways and on steps in the
winter?
4. Why is sodium chloride used rather than sucrose?
Teacher's Notes
When a substance freezes, the particles arrange themselves into an orderly pattern. This
arrangement is called a crystal. When sodium chloride is added to the water, a solution is formed.
The forming of the solution interferes with the orderly arranging of the particles in the crystal.
Therefore, more kinetic energy (heat) must be removed from the solvent (water) for freezing to
occur. This results in a lower freezing point. Furthermore, the more particles of solute (salt)
added, the more kinetic energy must be removed. The greater the concentration of solute, the
lower the freezing point of the solvent.
Answers to Questions
1. Sodium chloride is added to the ice to lower the freezing point of the ice.
2. Large crystals dissolve more slowly than small crystals. This allows time for the
ice cream to freeze more evenly.
3. When sodium chloride is placed on the highway or on steps, the freezing point is
lowered, and the ice melts.
4. Sodium chloride is used for three reasons. First, some solids such as sugar do
not dissolve in ice water as well as salt. Second, salt is an abundant mineral in
the form Halite and is not expensive. Finally, when sodium chloride dissolves, it
separates into two particles (Na+ and Cl-), lowering the freezing point further.
Only advanced students would need to know this concept. It is called ionic
dissociation.
Disposal
The ice/salt mixture can be poured down the sink. Ziploc™ bags can be washed and reused.
Credit: The formula for the ice cream mixture is from Mr. William M. Black of Kewanee High
School in Kewanee, IL.
http://www2.ncsu.edu/ncsu/pams/science_house/learn/CountertopChem/exp15.html
Countertop Chemistry Experiment 15
Ice Cream
Adding a solute to a solvent lowers the freezing point of that solvent. This change in freezing
point is referred to as a colligative property. In this experiment, you will use the lowered freezing
point of water to chill another mixture (ice cream) to the solid state.
Materials
Subsitutions
1 quart Ziploc™ bag
1 gallon Ziploc™ bag
1/2 cup milk
1/2 cup whipping cream
1/4 cup sugar
1/4 teaspoon vanilla flavoring
sodium chloride
rock salt or KCl
ice
thermometer
measuring cups (1, 1/2, and 1/4 cups)
Styrofoam™ cups
plastic spoons
Procedure
1. Into a one-quart Ziploc™ bag, place 1/4 cup sugar, 1/2 cup milk, 1/2 cup
2.
3.
4.
5.
6.
7.
8.
9.
whipping cream, and 1/4 teaspoon vanilla (4-hydroxy-3-methoxybenzaldehyde).
Securely seal the bag and mix well.
Into a one-gallon Ziploc™ bag, place 2 cups of ice.
Using the thermometer, measure and record the temperature of the ice.
Add between 1/2 and 3/4 cups of sodium chloride to the gallon bag.
Place the sealed quart bag into the gallon bag. Close the larger bag securely.
Holding the large bag by the top seal, gently rock the bag from side to side. Do
not hold the bag in your hands—it will be cold enough to cause tissue damage to
your hands.
Continue rocking the bag until the contents of the quart bag have solidified (1015 minutes).
Measure the temperature of the salt/ice mixture in the gallon bag and record the
temperature.
Remove the frozen contents from quart bag into Styrofoam™ cups. Consume the
contents of the cups.
Data and Observations
Initial temperature of ice _____
Final temperature of ice mixture _____
Change in temperature _____
Questions
1. Why is sodium chloride added to the ice?
2. Why are large crystals of sodium chloride used instead of small crystals
3. Why is sodium chloride placed on icy patches on highways and on steps in the
winter?
4. Why is sodium chloride used rather than sucrose?
Teacher's Notes
When a substance freezes, the particles arrange themselves into an orderly pattern. This
arrangement is called a crystal. When sodium chloride is added to the water, a solution is formed.
The forming of the solution interferes with the orderly arranging of the particles in the crystal.
Therefore, more kinetic energy (heat) must be removed from the solvent (water) for freezing to
occur. This results in a lower freezing point. Furthermore, the more particles of solute (salt)
added, the more kinetic energy must be removed. The greater the concentration of solute, the
lower the freezing point of the solvent.
Answers to Questions
1. Sodium chloride is added to the ice to lower the freezing point of the ice.
2. Large crystals dissolve more slowly than small crystals. This allows time for the
3.
4.
ice cream to freeze more evenly.
When sodium chloride is placed on the highway or on steps, the freezing point is
lowered, and the ice melts.
Sodium chloride is used for three reasons. First, some solids such as sugar do
not dissolve in ice water as well as salt. Second, salt is an abundant mineral in
the form Halite and is not expensive. Finally, when sodium chloride dissolves, it
separates into two particles (Na+ and Cl-), lowering the freezing point further.
Only advanced students would need to know this concept. It is called ionic
dissociation.
Disposal
The ice/salt mixture can be poured down the sink. Ziploc™ bags can be washed and reused.
Credit: The formula for the ice cream mixture is from Mr. William M. Black of Kewanee High
School in Kewanee, IL.
Vanillin (4-hydroxy-3-methoxy-benzaldehyde)
Table vs Rock Salt for Ice Cream
5/20/2003
Question - Rock salt is used for making ice cream. We have found table salt actually lowers the
temperature of ice water several degrees more. Why is table salt not used?
-------------------David,
It works quite well because it is more soluble than rock salt. It is not used much because it costs
quite
a lot more than rock salt.
Regards,
ProfHoff 669
=====================================================
No reason. It "works" better because table salt is a much finer grain and so it comes to
equilibrium with
the ice/water quicker. The only "ice cream maker's" reason might be that you do not want the
temperature
to get too low. There is probably some differential in price, but both forms of salt are so
inexpensive
for home-made ice cream compared to the other ingredients, that is not an important
consideration.
Vince Calder
=====================================================
I am not by any means an expert on making ice cream, but I am willing to make a guess. If it is
correct
that table salt lowers the temperature even more, the water might freeze faster and create ice
crystals in
the ice cream. I doubt most people want gritty ice cream. You could try an experiment--try one
batch with
table salt and one batch with rock salt and see what happens.
Vanilla
Ice
Cream
Preparation Time: 20 Minutes
Servings: 1-1/2 quarts
Ingredients:
2 cups half and half
One 12-oz can evaporated milk
3/4 cup sugar
1 teaspoon vanilla
1/4 teaspoon Morton® Table Salt
Morton® Rock Salt for Making Ice Cream*
Crushed ice*
* Refer to your ice cream maker instructions for specific amounts of rock
salt and ice.
Preparations
Combine ingredients; mix well. Pour into freezer container. Freeze according to
Freezing Directions.
Follow manufacturer's instructions for assembling ice cream maker. Fill freezer
tub with a mixture of one (1) part Morton® Rock Salt for Making Ice Cream and
eight (8) parts crushed ice to a level well above the line of the mixture in the ice
cream container. Pack ice down firmly and let stand 5 minutes. For a handcranked model, turn crank slowly, increasing speed as the mixture begins to
freeze. When the ice cream is frozen, the crank will be difficult to turn. Electric
ice cream makers will shut off automatically. To harden the ice cream, remove
the dasher; seal container. Drain off ice and salt mixture; repack freezer tube
with a mixture of three (3) parts crushed ice to one (1) part rock salt. Let stand
at least 1 hour.
Chocolate Cookie Variation: When ice cream is finished, remove dasher. Stir in
1 cup crushed chocolate sandwich cookies. Harden as directed
Pat Rowe
=====================================================
Rock salt is cheaper.
Some ice cream makers do recommend using table salt, so there is obviously no operational
reason not to use it. But if rock salt will suffice, using table salt in an ice cream freezer is like
washing your floor with distilled water - too much added cost for not much added benefit.
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
Rock salt forces the ice surrounding the can of ice cream mix to
melt. The "brine solution" or liquid that forms in the wooden
bucket absorbs heat from the mix and gradually lowers the
temperature of the mix until it begins to freeze. If there were no salt
added to the ice, it would melt at 32 degrees Fahrenheit and
eventually the ice water and mix would come to equilibrium at 32
degrees. The ice cream mix, however, does not begin to freeze until
its temperature falls below 27 degrees. Therefore, in order to freeze
the mix, we need a salt concentration, or a ratio of 5 cups of ice to 1
cups of salt. At this concentration, our brine temperature should
remain constant at 8 to 12 degrees F. This will give the rapid
cooling and freezing that is essential to making smooth creamy ice
cream.
More detailed information provided by David Winer, Bethesda,
Maryland For ice to melt into liquid water it must absorb a lot of
heat energy. In an ice cream maker this heat needed to melt the ice
comes from the mix, and so the mix chills. If the ice were not
combined with salt, the mix’s cooling would be slow and would
stop when the mix reached the temperature of melting freshwater
ice-- at 32 degrees. But salt makes ice melt faster, and drops the
temperature to that of freezing salt water. The heat used to melt the
ice is drawn much faster from the surroundings, which in this case
consist of the melted ice AND the ice cream mix. Since the
surroundings lose large quantities of heat to the ice, they cool
rapidly and continue to cool until they reach the freezing
temperature of melting saltwater ice. This temperature is below the
cream mixture's freezing point.
Rock salt forces the ice surrounding the can of ice cream mix to melt. The "brine
solution" or liquid that forms in the wooden bucket absorbs heat from the mix and
gradually lowers the temperature of the mix until it begins to freeze. If there were
no salt added to the ice, it would melt at 32 degrees Fahrenheit and eventually
the ice water and mix would come to equilibrium at 32 degrees. The ice cream
mix, however, does not begin to freeze until its temperature falls below 27
degrees. Therefore, in order to freeze the mix, we need a salt concentration, or a
ratio of 5 cups of ice to 1 cups of salt. At this concentration, our brine
temperature should remain constant at 8 to 12 degrees F. This will give the rapid
cooling and freezing that is essential to making smooth creamy ice cream.
More detailed information provided by David Winer, Bethesda, Maryland For ice
to melt into liquid water it must absorb a lot of heat energy. In an ice cream
maker this heat needed to melt the ice comes from the mix, and so the mix chills.
If the ice were not combined with salt, the mix’s cooling would be slow and would
stop when the mix reached the temperature of melting freshwater ice-- at 32
degrees. But salt makes ice melt faster, and drops the temperature to that of
freezing salt water. The heat used to melt the ice is drawn much faster from the
surroundings, which in this case consist of the melted ice AND the ice cream mix.
Since the surroundings lose large quantities of heat to the ice, they cool rapidly
and continue to cool until they reach the freezing temperature of melting
saltwater ice. This temperature is below the cream mixture's freezing point
.
http://www2.ncsu.edu/ncsu/pams/science_
house/learn/CountertopChem/index.html
Great website